A tractor trailer rig includes two vehicles: a tractor in which a driver sits, and a trailer pulled by the tractor. A supply cable extends from the tractor to a junction box connected to the front of the trailer. The supply cable includes electrical supply lines that conduct electricity from the tractor to respective breakers in the junction box. Distribution lines extend from the box into the trailer. They conduct electricity from the respective breakers to different devices of the trailer, such as a dome light and a brake system.